Time flies when you're having fun. But you're at work, and work sucks. So how is it 5:00 already?

When we talk about "losing time," we aren't referring to that great night out, or that week of wonderful vacation, or the three-hour film that honestly didn't feel like more than an hour. No, when we fret about not having enough time, or wonder where exactly all those hours went, we're talking about mundane things. The workday. A lazy, unremarkable Sunday. Days when we gave time no apparent reason to fly, and it flew anyway.

Why does that happen? And where did all the time go? The secret lies in your brain's ticking clock—an elusive, inexact, and easily ignorable clock.

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First of all, yes

In understanding any complex issue, especially a psychological one, intuition doesn't usually get us too far. As often as you can scrabble together a theory about how the mind works, a man in a lab coat will adjust his glasses, tilt forward his brow, and deliver a carefully intoned, "Actually..."

But not today. Most of what you think you know about the perception of time is true.

"Philosophers have written on [the perception of time] for a long period, and psychology has been interested in it since its inception as a separate discipline—since the late 1800s," explains Dr. Frederick M. Brown, Associate Professor of Psychology and Director of the Human Performance Rhythms Laboratory at Penn State University.

Brown says that common wisdom about how time "flies" is basically correct. "We do know that time can drag rather dramatically, and that when people get older, time seems to fly much faster." More broadly, "When people are interested in something, they almost escape the time dimension."

This jibes with common experience as well as decades of experimentation. What it doesn't do, though is explain that middle ground between bored and enthralled, when we're not having fun, but we're not consciously hating life, either. This time seems to fly, too.

The secret, Brown says, is something called habituation. This refers to the gradual adaptation to a stimulus or to the environment, "with a decreasing response," and explains why certain sensations—the bitterness of a lime, the pain of irritated rash, the shock of a loud sound—seem to diminish over time. But it can also help explain, says Brown, how you can sometimes "escape the dimension of time"—by essentially losing awareness of it.

"If there is a very regular noise—one that's observable but is not terribly noxious—after a while we tune it out." The hum of your computer, the drone of A/C, and muzak can all fade into the background. The task of listening to these items defers to the thing that you are intentionally doing, in terms of your attention.

The very act of thinking about time—How long until lunch? How long have I been here? I'm so bored!—has the tendency to make it feel as though it's passing more slowly. "A watched pot never boils" is particularly apt here; while it's certainly boring to watch water boil, the fact it has been colloquially codified as a Boring Thing makes it even more boring. In this scenario, the importance of the acts ranks as follows: 1. You're thinking about how long this seems to be taking. 2. You're thinking about the water boiling. 3. You're boiling water.

Imagine you had a little counter in your head, an internal clock, which most people believe the brain has, in some form. To monitor the passage of time, you sort of have to monitor, or add up, and count and collect those little clicks. Right? You have to keep track of how many are going by. But if your attention is devoted to a different task, like the visual search task, then you sometimes will miss the clicks that come by.

On top of that, as Brown explains, the inverse is true. "When things get irritating, like maybe the drip of a faucet, you can actually become sensitized, then that becomes the focus of your attention, so it's almost like you're waiting for the next drip, and you're not losing your sense of consciousness of the time passing." Replace the "drip of a faucet" with "the obnoxiously menial task at hand," and you get the idea.

But why?

Now that we understand the (extremely crude) patterns of Time Flight, we're left with a more fundamental question: Why the hell do our brains work like this?

It's easy to study people's perception of time, and lots of people have. And they've found all kinds of weird stuff:

• Time passes more slowly to people who are consciously keeping track of it than to people who aren't. (The study informed half its participants that they'd be asked about the length of the experiment, but not the other.)

• Drugs can do all kinds of terrifying/awesome things to the perception of time. (c.f. Our college years.)

• Certain health conditions can do the same. (An epileptic seizure, for example, can feel as though its lasted a very long time, even if it was over in a matter of seconds.)

Here's the problem: There's no clear answer as to why these things affect the perception of time. We can talk broadly about concentration and focus, but we can't put a finger on what, in our brains, controls this kind of stuff. Yet. In a paper published in the Philosophical Transactions of the Royal Society of Biological Sciences journal, Marc Wittman explores dozens of possible causes, but arrives at this conclusion:

Despite the fact that time is an essential factor for understanding complex behavior, the processes underlying the experience of time and the timing of action are incompletely understood.

In other words, we don't really know why this happens.

But we have a pretty good idea of how. So next time you catch yourself wondering how it's five o'clock already, rest assured: whatever you were doing, and however long it seemed to take, at least you were giving it your undivided focus.

Original illustration by Gizmodo guest Artist Robert Grossman. You can check out more of his stuff over at his website. Oh, and special thanks to flickr user zoutedrop for his awesome, Creative Commons-licensed photo of a clock.